@TNV I would use a port scanner (nmap) to see whether a web server is running on the router, and on which port(s)

I didn't bother scanning the whole range. Just ports that I might get access

PORT STATE SERVICE

22/tcp closed ssh

23/tcp closed telnet

80/tcp closed http

443/tcp closed https

8008/tcp closed http

@TNV That nmap shows the ports closed, it seems like MAC filtering or IP filtering had previously been implemented. Either that, or access to the admin page has been disallowed on the WiFi interface, and only allowed via wired -- is that what you're doing?

Now that the correct firmware is definitely on the router, maybe try another device reset.

@IntuitiveNZ 

Thanks for your help. I appreciate it. 

MAC filtering has never been enabled.

Regardless, I'm wired into the LAN ports on the router.

I've done another hardware factory reset (Reset button for 10 seconds).

Same results.

It's frustrating because the correct firmware should be on there, and a factory reset should bring it back to life. Especially since it's partially alive.

That is frustrating. What I would do is install DD-WRT on it. Use the same method as before to load the firmware on it, but put the DD-WRT file in its place. https://dd-wrt.com/support/router-database/?model=Archer%20C9%20(AC1900)_1.x

I've been able to use TFTP to send the firmware to the router, but it doesn't seem to be applying the firmware. I tried to upload the DDWRT factory-to-ddwrt.bin and it didn't seem to take.

I can sort of check this by using nmap's host OS detection stuff. It doesn't seem like that is changing at all when I change the recovery firmware.

Extra tough to know what state the router is in because the LEDs are burned out.

@TNV 

If the serial number is not showing as valid, contact support as they can help add the serial number after verifying the device (copy of receipt, and picture of sticker with the serial number).